Distillation process



Feb. 6, 1945.v H, R. LEGATSKI DISTILLATION PROCESS Filed March 6, 1942 mmmzma zou mmbmmmma ON u y Patented Feb. 6, 1945 DISTILLATION PROCESS Harold It.r Legatsk, Bartlesville, Okla., assignor to Phillips Petroleum Company, `a corporation of Delaware Application March 6, 1942, Serial No. 433,667

l 3 claims. (C1. 19e- 94) l This invention relates to improvements inthe art of fractional distillation. More particularly it relates to the separation of selected fractions from mixtures containing substantial portions of Y material other than the desired material The separation of a mixture into its various components by fractional v'distillation means is well known in the art. Generally, in operating fractional distillation columns small variations in temperature, pressure, reflux ratio, and the like can be tolerated without appreciably affecting the quality of the overhead or other streams effluent from said column during steady operation. However, in some particular'separations where a relatively small amount of material is separated and isolatedby fractional distillation from a relatively large amount of undesired material admixed therewith and the boiling rangesV of the desired and undesired material are near each other, a relatively high reflux ratio is necessary to bring about the desired separation and very close control of conditions such as temperature, pressure, reux ratio, and the like is neces-l sary to produce narrow boiling range fractions containing an optimum amount of the desired material. e

Heretofore, the control afforded in operation of fractional distillation columns on a commercial scale has often not been precise enough to effect such separations and excessive amounts of undesired material were often admixed with a fraction of desired material even under heretofore optimum conditions of operation. Such lack of precise control particularly `applies to existing methods for the separation by fractional ldistilla. tion of close boiling fractions containing essentially normally liquid components. This applies particularly to material having a`boiling point above about 115 F. at'atmospheric pressure when cooling and condensation is effected by ordinary cooling water. l I have now found a new arrangement for effecting precise control in a fractional distillation system of the quality of a Adesired fraction by properly controlling in a novel manner the temperature and volume of reflux passed to a fractional distillation column and the pressure on said Another object of my invention is to separate and isolate fractions,of narrow boiling range of desired material, from Vmixtures containing said desired material in small amounts together with undesired material ofv closely adjacent boiling a desired normally liquid material from a mixaterial in a rela'- bons from mixtures containing them. It can beI successfully applied to the fractional distillationv of any mixture containing distillable components, such as organic mixtures which may comprise or contain alcohols, aldehydes, acids, organic halides, esters, ethers, amines, or any other materials capable of being fractionally distilled.

My invention-will now be described in connection with the accompanying drawing which is a flow diagram `illustrating a specic operation of a fractional distillation system. As has been column, so that not only can a constant reflux range from mixtures containing undesired ma terial.

. stated, my invention will find application in the separation of vmany kinds of materials. The drawing will be discussed in connection with the separation 'of a selected hydrocarbon fraction from a mixture of hydrocarbons.

A hydrocarbon mixture, comprising isohexanes y and higherboiling hydrocarbons `and essentially free `from C5 and lower boiling, hydrocarbons is passed to fractional distillation column through conduit Hl controlled by valve l l.

Fractionator l2 is operated to separate a lowboiling isohexane fraction from the mixture charged thereto, and may be any suitable fractional distillation column. The top of fractionator l2 is operated at predetermined conditions of temperature and pressure and such conditions are maintained substantially constant. An isohvexane fraction in the vapor state is passed over-` head through conduit I6, back-pressure control valve 2D, conduit Il cooler and total condenser 2li, and conduit I8 to reflux accumulator 25.

Valve '2li is for controlling the pressure in the column at a substantially constant predetermined value, and is responsive to the pressure in conduit f6, as diagrammatically illustrated. At least a portion of the liquid from reflux accumulator 25 is passed through conduit 43 by a pump lil andy is returned as reflux to the top of fractionator l2 through conduit d5 and flowcontrol valve 45. ,A low-boiling isohexane fraction, recovered as a product of the process, is removed from accumulator 25 through conduit 4l and level control valve 35, which is in communi cation with the levelA of the liquid in accumulatorI .A

through float mechanism 35.

Hydrocarbon material having a higher boiling range than that removed through overhead conduit I6 and comprising substantially any and all normal hexane together with any higher boiling hydrocarbon charged to fractionator l2 can ybe removed from the fractionator through conduit lll controlled byvalve l5. fractionator l2 by means of heating coil I3.

For the successful isolation of a low-boiling isohexane fraction of desirable quality and narrow boilingy range, such as through conduit al, the predetermined operating pressure and temperature for the fractionating column l2 and partic- Heat is supplied to ularly the top section of the coluinnrshould be. f

i maintained at essentially constant values during long periods of continuous operation.y Although the conditions may be at any value within a substantial range, once the operating conditions are selected, the temperature in the top ofthe column should not vary more than a fraction of one degree Fahrenheit, andthe pressure should not vary more than a fraction of one pound per square inch.

In order to exert such close control on temperature and pressure conditions in the top of fractionator I2 it'is important in my arrangement that essentially all of the overhead material passing through condenser 2B be completely condensed and cooled several degrees below its condensation temperature and that the material eilluent from condenser 26 and before it reaches the juncture with conduit 30 be at a temperature substantially lower than the temperature desired for the material in the liquid phase in accumulator 25. For a distillation such as is being particularly discussed, the cooling and condensing eiected by condenser 26 is normally obtained by the use o cooling water in an atmospheric cooling tower, and variations in atmospheric temperature and in the temperature of the cooling.v

water from one part of the day to another have heretofore varied the temperature of the material effluent from condenser 2B and, hence, the temperature of the c-ondensate which reached the accumulator, and of the reflux returned to the column. However, according tomy arrangement,

the temperature of the liquid in the accumulator 25 can be successfully controlled and maintained substantially constant by a temperature control valve 3| in conduit 30, which admits uncondensed vapor to the condensate in conduit I8.

ValveSl is actuated by a controller whichis responsive'to the temperature of the liquid phase S3 by means of lcommunicating means 32. When the temperature of the liquid phase 33rin. accumulator 25 tends to be less than a predetermined minimum value, said value being dependent upon the temperature of the reux desired in column i2, control valve 3| opens, thereby allowing uncondensed hot vapors from conduit I6 to pass through conduit 22 and conduit 30 to be admixed such an accumulator varied accordingly.

and liquefied with cooled material from condenser 26. Since I preferr to operate my fractionation system in such a manner that material directly efuent from condenser 26 is always at a temperature substantially lower than the temperature of the material inthe liquid phase in accumulator 25, valve 3| is opened to such an extent that some ,mat/arial always flows therethrough and particularly when steady state operation is'attained. The predetermined and desired temperature of the liquid phase in accumulator 25 can be attained when a minimum but appreciable amount-of vaporous material is flowing through valve 3l.

In some instances communicating means 32 may be arranged in my system so as to be responsive to the temperature in conduit l'l and in communication therewith at a position which is in the vdirection of the accumulator from the juncture of conduits 3@ and I8. My system can be arranged in the manner shown when the time of residence of liquid phase in accumulator 25 is relatively short, as is often the case.

The amount of reflux liquid passed by reflux pump te through conduit :l5 is dependent to some extent upon the pressure head on pump All which in turn is directly dependent `on the pressure existing in accumulator 25 von the surface of the vliquid phase. i

dangery that a large typevof such an accumulator would not be able yto withstand such a variation. In Aseveral particular instances, when a large accumulator in use as'discuss'ed was suddenly cooledfby atmospheric conditions,such as a rainstorm, it has collapsed before simple temperature control, as afforded by valve `3l alone,V could increase the vapor pressure in suchk an.

accumulator.

In order to insure a minimum pressure vari'- ation in accumulator 25, conduit 22 communi-v cates between conduit IE and the vapor space 34 in accumulator 25.y A pressure control valve23 is located'in conduit 22 and is responsive to the vapor pressure in the accumulator. When'the pressure in accumulator 25 tends to decrease below a predetermined va1ue, such as. by external cooling of the accumulator or zby othenmeans, the controller, being so adjusted beforehand, -opens valve 23 thereby allowing vapor at a higher pressure than the pressure lexisting in accumulator 25 to enter the vapor space of said accumulator, through conduit 22 above the surfacel of the liquid, thereby preventing the pressure in Y the accumulatorfrom decreasing. I prefer to operate my fractionatingsystem in sucha manner that some vapor material 'flows through valve 23'at all times during steady operation thereby maintaining a constant positive pressure on the surface-of the liquid in accumulator 25, and the pressure so, maintained being the predetermined and desired pressure in the accumulator.

This pressure will be slightly above the vapor 'pressure actual output which is desired during normal operation Using a pump having such a capacity, I prefer to employ a ow control valve 46 in conduit 45 which will regulate the amount of mate"- rial flowing therethrough so that said amount of material is substantially constant throughoutA continuous operation. The control of ow of reiiux liquidl in conduitk 45 is further facilitated,

however, by electingthe regulation of the pressure inthe accumulative as discussed. l

. VIn actual operation, by far the greater amount of vapors from the top of vcolumn I2 and conduit I6 willpass through -valve 20 and conduits I1 and I8sovthat variations in the amount passed through conduit 22 from conduit I6 will be relatively small and will not upset or appreciably aiect the pressure at the top of column I2.

The combined use of the arrangement described for maintaining a constant pressure on the fractionalv distillation column, a constant temperature of the liquid in the reux accumulator, and a constant pressure in the reflux accumulator provides very satisfactory operation when fairly extreme.v variations in the temperature of the cooling medium for the cooling and condensation, and also when the reflux accumulator itself may be subjected to rapid temperature changes. Both of these situations are often met in commercial operations, particularly in the fractionation ofl motor-fuel range hydrocarbons to produce ,fractions havingl a narrowboiling range, such as to 15 F. When a well insulated reflux accumulator is assured, the use of the pressure control feature, whereby vapors are directly injected into the vapor space of the accumulator, may not :be necessary; and when a constant temperature `for the condensate is assured the temperature control feature, whereby the cooled condensate is `warmed by admixture with uncondensed vapors, may not be necessary, as will be appreciated by those skilled in the art.

The following exampley illustrates av partieular adaptation of my invention although it is not my intention to be unnecessarily limited in the scope of my invention by the specific arrangement and conditionsdiscussed therein.

EXAMPLE A hydrocarbon mixture containing substantially no hydrocarbons lower boiling than C1 hydrocarbons is charged to fractionator I2 through conduit l0 controlled by valve II. Such a mixture has the composition shown in Table I.

By suitable adjustment of pressure control valve and its cooperation with control valves 3I and 23 the pressure in the accumulator 25 is ymaintained at a substantially constant value of 8 pounds per square inch gauge. Variation from this value is only a fraction of one pound per square inch. The accumulator temperature is maintained substantially constant at 198 F. Bythevclose control aiorded by the cooperation of` valves 20, 3| and 23 as discussed herein, variationfrom this value is less than one degree Fahrenheit. `Thekettle of fractionator I2 is operated at a temperature 0f 306 F. and a pressure ofv30pounds persquare inch gauge. With a reux ratio in the fractionator of 25 to 1 the temperature of the head of thev fractionator is maintained within a fraction of a degree of 232 F. and by proper control of valve 46 in cooperation with valves 20, 3l and 23 the pressure in the head of the fractionator is maintained within a fraction of a pound of 22 pounds per square inch gauge.-

Approximately 90,718 gallons per` day of charge stock of the stated composition is passed to fractionator I2 ythroughconduit I0, Characteristics of this charge stock are shown in column 1 of `Table II. An isoheptane fraction in the vapor state and of high octane number is passedfrom the top of fractionatorlI2 through conduit I6, pressure` control valve 20, cooler and condenser`26, and conduit II to reflux accumulator 25. A liquid from reflux accumulator 25lis passed through conduits 43 and 45 by pump '44 and thence to the top of fractionator I2 through valve 46. By using a reflux ratio of 25:1 van isoheptane fraction of narrow boiling range and other desirabley characteristics is recovered asa product of the process through conduit 4I controlled by valve tionator I2 constitutes approximately 35,700 gallons per day and has the characteristics as shown in column 2 of Table II. t

' Table II. i

Charge to Isoheptane vKettle ractionator 12 fraction product A. r. I. gravity 59.1 60.0 v 57.9 Boiling range F.1. 185-456 l69l83 214-454 Reid vapor pressure 2.0 3. 17 1.05 A. S. T. octanelnumbe l Clear 57. 4 76 45 3 cc. TEL 70. 6 89 58. 9

Boiling range under atmospheric pressure of 13.2 pounds per square `inch absolute.

The kettle product from fractionator I2 amounts to approximately 55,018 gallons per day and comprises normal heptane and heavier hydrocarbons which are removed from the fractionator through conduit I4 controlled by valve I5. Such a. kettle product has characteristics as shown in column 3 of Table II.

In the separation of an isoheptane fraction of such magnitude as exemplied from higher boiling material, conduit I6 has a diameter of yap` proximately 24 inches and conduits 22 andl 30 have diameters of approximately 6 inches.

It is to be understood that my process can be applied to any fractional `distillation where the overhead vapors are subjected to total condensation. Many modifications of this invention may obviously be used, and can be` adopted by one skilled in the art without departing from the spirit of the disclosure. The restrictions used in the example and in connection with the drawing, need not necessarily be used as limits for any particular operation or set of conditions as they are presented primarily as illustrative examples., Itvwill be `understood that the flow diagram presented and described herewith is The isoheptane vfraction removed from fracdilation,

schematic only and that other additional pieces distillation column, removingk from the topv of saicll v'column a vapory containing at least one component of said vmixture, p'assinflg; a major portion vof said vapor through aback pressure control valve to a cooler and condenser, vconldensing all of saidmajor portion of 4said' vapor 1and cooling the resultant condensate toy a tem'- lperature below that subsequently desired-for a `reiiuxl to said column, admixing with theoresultant condensate a'minor portion of saidl vapor in lan amount such as to produce a resultant liquid materialat a desired temperature, pass `ing saidv liquid to an accumulator y*containing a liquid space and a vapor space, passing to said vapor space a further minor portion of said vapor inA an amount such as to maintain a desired essentially constantl pressure in said accumulator `greater than the vapor pressure of the liquid 1 contained therein,` passing a portion of the'liquid `from said accumulator to the top of said fractiona1 distillation column as a reflux for said ycol'umn, and recovering a further portion of the liquid from said accumulator kas a product of the distillation; y

2. In a process for continuous fractional disthe improvement which comprises completely condensing a major portion of a va- ;porous overhead eiiiuent of a fractional distillation column to form a condensate, passing the resultant condensate to a reflux accumulator having va liquid space and a vapor space, passling a portion of the liquid from said reux accumulator to the top of said fractional distil- 3lation column as reflux, controllably injecting into said condensate a minor portion of. said vaporous effluent to maintain 'the Vresultant mixture at a constant desired reflux tempervature, and controllably injecting into the Yvapor space V of said" accumulator a further minor amount oftsaid vaporous elluent 'to maintainl the resultant pressure at a constant value higher than the vvapor` pressure of they condensate.

3. A process lfor the continuous fractional dis-` tillationy of a low-boiling normally liquid hydrocarbon fmixture to obtain a desired hydrocarbon fraction boiling in the motor fuel range and` having va boiling range not greater than about 15 F. which comprises, continuously charging to a fractionaldistillation column such a hydrocarbon mixture essentially freeofhydrocarbon components lower boilingthan saiddesired lfraction to'lbe obtained; continuously removing from the top of said columna ivapor fraction corresponding to Ysaid desired fraction, completely condensing a majorA portion of said vafpor vfraction to form a condensate and cooling said condensate, to a temperature lower than that subsequently desired for a rei'luX, injecting into said cooled condensatea controlled amount of a minor portion of saiduncondensed vapor fractionto form a liquid hydrocarbon fraction vat va constant and desiredv reuxv temperature,

passing said liquid-fraction to anaccumulator, maintaining in said accumulator a liquid level such that a substantial vapor space exists above the liquid fraction contained ltherein, continuously passing a further controlled minor portion of said uncondensed vapor fraction to said vapor space to maintain therein a constant and desired pressure, higher than the vapor pressure Aof* the liquid'contained in said accumulator, con` tinuously passing a substantial' and controlled portionof said liquid `fraction of essentially constant temperature and constant pressure `from said accumulator'tothe top of said column as refluxyandremoving ajfurther portion of said liquidgiraction as a desired hydrocarbon product ofsaiddistillation,

' c j AHAROLD R; LEGATSKI. 

